This is a companion work to the 1810 Federal and the 1818 Territorial censuses published in the Illinois Historical Collections. This Illinois state census lists the population by counties in 1820 and contains notes comparing discrepancies between names as written in the 1818 Territorial, the 1820 State, and the 1820 Federal censuses. There are over 25,000 records which indicate the following information: the name of the head of the family, number of males under 21, number of males age 21 and up, number of females under 18, and number of females age 18 and up.